The primary objectives of this study are to compare the safety, tolerance, virologic effect and immunologic effects of four treatment regimens: d4T, nevirapine, ritonavir (Arm A), d4T, 3TC, nelfinavir (Arm B), d4T, nevirapine, nelfinavir (Arm C) and d4T, 3TC, nevirapine, nelfinavir (Arm D) in HIV-infected children. Secondary objective include evaluation of the response to therapy as measured by growth velocity, changes in neurodevelopmental indices, and HIV disease progression based on time to development of the first new CDC category C diagnosis. Estimates of exposure to study drugs, based on population pharmacokinetic parameters, will be correlated to virologic responses. Lymphocyte proliferation responses to recall mitogens, microbial recall antigens, and HIV antigens will be evaluated across treatment arms and correlated with virologic inhibition. The study has been fully enrolled.